• In August I paid £39 for a Groupon deal that appeared to offer a full service for my gas boiler through a company called Homecall+. The money was deducted from my credit card the next day, and ever since I have been attempting to get a booking for Homecall+ to fulfil the deal.

Initially I used the online booking form (as stated on the voucher), and waited more than the 10 days it says it will take them to respond. I did this twice, then went away for three weeks in September.

On 3 October I re-applied and again nothing happened. The Homecall+ website gave the email address of a David Needham to contact. This I did but received no response. I contacted the company by phone – the 0800 number went to a voicemail which was full so I could not leave a message.

I complained to Groupon, which simply gave me the email and phone number I had already tried.

This is the second bad experience I have had with Groupon. I'd earlier purchased teeth whitening, which cost £119. On the day of the appointment, the teeth people called to say the booking was cancelled. When I asked why, I was told to speak directly to Groupon. I have now resolved the teeth whitening issue with the vendor but I'm still no nearer getting my boiler serviced. JL, London

• I recently purchased a Groupon voucher for a full gas boiler service and inspection for £39 with Homecall+ (the claimed value was £125). I arranged for the engineer to come on 10 October and was asked to be at home between 8am and 6pm. The engineer finally turned up at 7.30pm, after having worked, he said, the whole day from 8am.

The only problem is that he managed to leave with my boiler leaking gas. As a result I had to call out a local guy who luckily came and fixed it. I have tried to contact Homecall+ but they are not answering the phone. Groupon told me they would look into the matter, as it seems that lots of other people seem to be having problems with Homecall+. PW, Leicester

• I used Groupon to book two teaching sessions at the SNO!zone in Milton Keynes at a cost of £180 for my family. I was informed at the time that the lessons could be taken up to 1 December and it seemed like a great deal. On attempting to book the lessons in early October, I was told by SNO!zone that there were no sessions available in the evenings or weekends until after the offer expiry date. The only lessons were during the week. This was due to what they said was 'unprecedented demand'. Given that my vouchers were for a family lesson, and my children are of school age, it seems unreasonable to insist I illegally take my kids out of school to redeem the voucher. Three weeks later and I have had no response from the Groupon customer services email address and I have yet to get through on their phone line. RF, London

We've been getting an increasing number of complaints about Groupon offers, and Homecall+ was a name that immediately rang bells with us.

For those of you new to the Groupon phenomenon, it is a website that sells discounted offers on a range of goods and services. The offers include beauty treatments, meals out, cut-price hotel deals and, as we've now seen, boiler servicing.

Groupon gets together with retailers that want to boost their businesses quickly by getting people through the door. A beauty parlour, for example, might offer 100 facials at a discounted price, usually at least at (a claimed) half price, and if 100 people sign up the deal goes ahead.

Consumers pay Groupon, which takes its cut and passes on the rest to the retail partner. A voucher is produced, and the hope is that the customer will spend more money with that retailer in the future.

The company has been a massive internet hit, and made the news last week when it floated on the New York stock exchange. The share offer valued the company at an incredible £7bn, which some might think is amazing given that it is yet to generate a profit, despite having more than 116 million subscribers.

Groupon operates all over the world, and when the UK-based operation works well, everyone is happy. However, as the site has grown, an increasing number of problems have arisen. These usually come about because the retailer cannot cope with the demand, or the partner company ceases trading or goes back on the original offering because it priced it too low, and then refuses to honour it. Vouchers generally have to be used by a certain date, and if you fail to do so you lose your money. Stores not wanting to honour the deal can put barriers in place, and wait for the vouchers to expire.

The big obstacle has been getting Groupon to deal with problems, as these cases show. Usually, Groupon offers a refund or voucher credit, but getting through to the firm, consumers say, can involve long (20-minute) waits on the phone. Emails to the company can go unanswered for weeks.

Groupon UK has told us that it takes great care when choosing its partner companies but, as the boiler stories above show, it perhaps needs to take a bit more care that its partners can deliver on deals.

The advertising blurb on the Groupon site for the Homecall+ offer describes how the firm has "carved a reputable name in the insurance industry". Quite how it came to this conclusion is anyone guess, because last May the company, in its previous incarnation, went bust leaving thousands of households with worthless boiler insurance policies for which they had paid about £130.

The official receiver was called in, and the assets of the then Blackburn-based company were reportedly bought by an entrepreneur called Lauren Dalton, who appears to have done the deal with Groupon. We say appears, because it is currently impossible to get hold of anyone at Homecall+. The number is not being answered, nor are emails, and Groupon does not seem to be able to get hold of anyone at the firm either.

We asked Groupon about these three cases and it issued the following statement: "Groupon continually strives for high standards of excellence to ensure that every experience we offer to our customers is of the highest quality and fully meets their expectations.

"We operate a stringent vetting process and have in place a number of quality controls to ensure all of our advertised experiences meet with these standards. Groupon has taken steps to resolve the issues with the partners involved to ensure the remainder of the promotion is in line with our standards and offers customers the experience they expect."

It has offered both JL and PW refunds, and in the latter's case an extra £40 to cover the cost of the repairs. PW says he will now employ a local plumber to service his boiler, which he wishes he had done in the first place.

Since our intervention, SNO!zone has been able to find some extra booking capacity, and as a result, RF will now be able to use his vouchers and get the family ski lessons he bought.

To clarify the legal position, we asked consumer law expert, University of Hull's Christian Twigg-Flesner, whether Groupon is ever liable for the failings of its partner companies. He told us the company has been structured in such a way that the partner firms retain all liability for the services they provide.

Groupon has an obligation to provide the voucher, he says, but once it is handed over to the partner supplier, the quality of the goods or services is then a matter for the relevant provider. If the voucher is not honoured, then it is a Groupon problem, he says.

Interestingly, Groupon was in hot water in China this week after it linked up with a company selling counterfeit luxury watches, suggesting it still has work to do in quality controlling its partner offers.

In the meantime it leaves consumers with a bit of a dilemma: to use Groupon or not? Ultimately, whether Groupon is value for money will depend on what it is offering in your area, and whether you would buy the items anyway. Some of the prices look attractive, but one wonders whether anyone ever pays the full price quoted.

We'd like readers who have used the site to email us with their experiences – good, bad or indifferent – and we'll report back.

We welcome letters but cannot answer individually. Email us at consumer.champions@guardian.co.uk or write to Brignall & King, Money, the Guardian, 90 York Way, London N1 9GU. Please include a daytime phone number